1. Field of the Invention
This invention relates to a polyvinyl chloride composition having improved processability, high clarity, and high heat distortion temperature.
2. Description of the Prior Art
Polyvinyl chloride is in general use because to its outstanding chemical and physical properties. Nevertheless, it is not necessarily satisfactory in heat stability and impact strength, and it requires improvement in processability and heat distortion temperature. In order to overcome these disadvantages, there have been proposed a variety of solutions, including the addition of stabilizers, plasticizers, lubricant, and other chemicals. Improvement through polymeric blending is also commonly practiced. Polymeric blending accelerates gelation in the molding process of polyvinyl chloride, permits deep drawing of moldings, and improves heat distortion temperature of moldings.
There is disclosed in Japanese Patent Publication No. 5311/1965 a polymer consisting mainly of methyl methacrylate which is claimed to accelerate the gelation of polyvinyl chloride and improve processability wuch as an increase of elongation at high temperatures. According to this disclosure, the polymer should have a specific viscosity .eta..sub.sp greater than 3 (as measured a 1 g/1000 ml chloroform and 20.degree. C.). One to three parts of this polymer is added to 100 parts of polyvinyl chloride to greatly improve the processability in rolling and to give good sheet molding. However, it was found that if it is added in larger quantities, for example 5 to 10 parts, the resulting blend becomes extremely viscous in the melt flow, with the result that mixing resistance increases and molding is very difficult to perform. Moreover, using this process, moldings are tinged with a yellow color, and remarkable shrinkage of molding also takes place.
There is disclosed in Japanese Patent Publication No. 29895/1976, incorporation of polyvinyl chloride with a methyl methacrylate-alpha-methylstyrene copolymer. This blending improves the heat distortion temperature of polyvinyl chloride, but has a serious drawback in that ungelled particles occur in large numbers in sheets extruded from such blend. Moreover, due to the lack of lubrication on a roll mill, the blend is liable to stick to the screw, die and calendering rolls used therein, making difficult, continuous operation. These disadvantages are pronounced when high speed molding is carried out at a high molding temperature.
Thus, there exists in the art a deficiency which needs to be fullfilled.